(1) Field of the Invention
The invention relates generally to the field of systems and methods for performing digital signal processing operations in connection with signals and more particularly to systems and methods for characterizing signals to determine their stochastic properties, that is, to determine whether they are random. More particularly it relates to a system for performing this function of characterizing signals that represent information on small samples, which in turn is representable as a composite of four component items of mutally orthogonal measurement information. If the signals are random, they may be determined to constitute noise, in which case additional signal processing efforts which might be undertaken to process the signals to extract information therefrom can be avoided. Stated another way, the system and method allows a determination to be made of the extent to which a pattern of data items, or sample points representing four dimensions of measurement information conforms to a random structure of data.
(2) Description of the Prior Art
In a number of applications in which four mutually orthogonal items of measurement information undergo processing, it is desirable to be able to determine the likelihood that a signal is random. For example, an acoustic signal, received in an ocean environment, may constitute noise alone, or it may include some useful information along with a background noise. If the signal constitutes noise alone, its amplitude will be random, but if it includes information it will not be random and further processing may be useful to identify the information. In some prior art signal processing systems, it is assumed that four mutually orthogonal items of useful measurement information are present in the signal, and the signal is processed to try to extract this intelligence. It may be the case that the noise level of a received signal is so great that the information cannot be extracted and the processing effort will be wasted in any event. It is accordingly desirable to be able to determine the likelihood that a signal constitutes only noise, or if it also includes four mutually orthogonal items of measurement information so that a determination can be made as to whether processing of the signal to extract the information would be useful, particularly when such four-dimensional data are sparse in quantity, i.e., a small sample of measurements are available for processing the signal.
The availability of four dimensional tracking systems, comprising the processing of three sensor-based measurements and time as a fourth dimension, is well known to those skilled in the art. One such reference, hereby incorporated in its entirety, is the technical paper xe2x80x9cThree-Dimensional Tracking Using On-Board Measurments,xe2x80x9d C. M. Rekkas, et al., IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS, Vol. 27, No. 4, July 1991.
A commonly assigned herewith U.S. Pat. No. 5,963,591 issued Oct. 5, 1999 discloses a system and method to characterize whether randomness is present in signal samples representable as a composite of four embedded orthogonal signal data items. One illustrative use of such a system and method is in processing underwater sound signals in connection with submarine undersea warfare, in order to spatially localize the source of emitted sound signals from a sonar contact received by a submarines towed sonar array. As a practical matter there are a number of conditions which cause data spareness, including:
(i) Extremely low data-rate (20-sec/datum in most underwater naval applications);
(ii) Naval tactical strategies require rapid maneuvering, thus data is lost in transitions;
(iii) Measurements corrupted by environmental background noise and other interferences;
(iv) Transient behaviors of underwater signals (launch signatures, sonar frequency, etc); and
(v) Imperfection in physical devices.
There are a significant number of practical situations where it is desireable to process collections of signal data too sparse to yield a determination of whether or not the signal is solely random noise by the xe2x80x9cnearest neighborxe2x80x9d methodology of processing taught by U.S. Pat. No. 5,963,591. Accordingly, there has been a continuing need to provide a system and method having improved capability for characterizing whether randomness is present in sparse accumulations of signals which are composited of four orthogonally related signal data items. Other prior art patents addressing systems and methods for characterizating whether randomness is present in data samples includes U.S. Pat. No. 5,966,414 issued Oct. 12, 1999, and U.S. Pat. No. 5,703,906 issued Dec. 30, 1997. (These are commonly assigned herewith also). However, none provide the teachings to address this need for characterization of presence of randomness with embedded four orthogonally related data items under conditions of sparseness of date. Likewise, an article which the inventors hereof co-authored with another co-author xe2x80x9cNovel Method for Characterizing Stochastic Processes and Its Application in the Undersea Environmentxe2x80x9d, Proceedings of the 6th International Conference on Signal Processing Applications and Technology, June 1995 does not contain disclosure of teachings to address this need.
It is therefore an object of the invention to provide a new and improved signal processing system for processing signals which may contain useful information comprised of four mutually orthogonal items of measurement information to determine the stochastic (random) properties of the signals based on small (sparse) data.
As a brief summary, the signal processing system processes a digital signal, generated in response to an analog signal which includes a noise component and possibly also another component consisting of four mutually orthogonal items of measurement information. An information processing sub-system receives the digital signal and processes it to extract the information component. A noise likelihood determination sub-system receives the digital signal and generates a random noise assessment indicative of whether the digital signal comprises solely random noise, and also a degree-of-randomness assessment indicative of the degree to which the digital signal comprises solely random noise. The operation of the information processing sub-system is controlled in response one or both of these assessments. The information processing system is illustrated as combat control equipment for submarine warfare, which utilizes a sonar signal input produced by a towed linear transducer array, and whose mode of operation employs four mutually orthogonal items of measurement information comprising: (i) clock time associated with the interval of time over which the sample point measurements are taken, (ii) conical angle representing bearing of a passive sonar contact derived from the signal produced by the towed array, (iii) a frequency characteristic of the sonar signal and (iv) a measurement of the signal-to-noise ratio (SNR) of the sonar signal.